1. Field of the Invention
The present invention relates to a vehicle anti-theft system, which limits theft by sensing a tilt of a vehicle using a tilt sensor (e.g., an acceleration sensor) to warn. Also, the present invention relates to an anti-theft system for a four-wheel vehicle and a two-wheel vehicle.
2. Description of Related Art
Conventionally, anti-theft systems, which limit theft of a vehicle due to invasion, a window breakdown, and jack-up, have been available. With regard to the jack-up, Japanese Unexamined Patent Publication No. H4-215547 discloses a vehicle anti-theft system, which includes a tilt sensor for sensing a tilt of the vehicle using a vibration detector for the vehicle. Here, the vibration detector includes a pendulum and an optical element.
The vehicle anti-theft system disclosed in Japanese Unexamined Patent Publication No. H4-215547 detects a characteristic of pulses based on a pulse number per unit time and a pulse width of the pulses outputted by the vehicle vibration detector. Then, the vehicle anti-theft system compares the detected characteristic with a predetermined determination rule to determine whether the outputted pulses indicates behavior of the theft or of another. Thus, erroneous determination can be limited.
However, the vehicle anti-theft system disclosed in Japanese Unexamined Patent Publication No. H4-215547 may not cope with various parking environments, because the above determination rule is predetermined.
For example, when the vehicle is parked under a severe environment, in which the vehicle is frequently vibrated (e.g., a side of a major road, a side of a construction site, a simply-built multistory parking area), the vehicle vibration detector may detect vibration due to the environment. In this case, the pulse outputted from the vehicle vibration detector is also compared with the predetermined determination rule. If the pulse has a characteristic that can be regarded (determined) as the behavior of the theft, it would be determined as the possible theft every time the vehicle is vibrated, although the vibration is not related to the theft. Therefore, a warning of a horn or siren may be repeatedly operated and thereby annoying neighbors.
A determination rule, which would not cause the above erroneous detection, is preferably required. However, from a viewpoint of the vehicle anti-theft, the warning should be operated as often as possible if the situation is suspected to be the vehicle theft (i.e., if the situation indicates the possible theft). Also, the anti-theft of the vehicle would not be realized if the determination rule were merely mitigated.
Some automobiles and auto bicycles include an electronic control unit (ECU) dedicated to a vehicle anti-theft. This ECU has a tilt sensor in order to reduce the theft of wheels or the vehicle itself due to the jack-up. Further, recently, it is suggested that other sensors (e.g., an acceleration sensor that is mounted to a vehicle for other purpose than the anti-theft use) may be used for the anti-theft purpose. This causes a cost reduction because the ECU dedicated to the vehicle anti-theft is not required.
For example, Japanese Unexamined Patent Publication No. 2004-243806 corresponding to US Patent Publication No. 2004-217849 discloses that various acceleration sensors, which are mounted to the vehicle for other purpose than the anti-theft use, are used for the original purpose and also for the anti-theft purpose. Specifically, when a sensitivity of the sensor is changed from a first sensitivity to a second sensitivity during a period, where the anti-theft command for the anti-theft operation is given. Here, the first sensitivity is used for the original purpose, and the second sensitivity is different from the first sensitivity. Then, it is determined whether the vehicle is under a threat of the theft based on whether the acceleration (tilt angle) detected by the acceleration sensor is larger than a predetermined threshold vale or not.
However, because the anti-theft system of the Japanese Unexamined Patent Publication No. 2004-243806 uses a constant threshold valve (fixed at 0.1 G) for determining whether the vehicle is under the theft or not, erroneous detections may occur, and also the detection may delay. When the vehicle is parked at a flat surface, which is flattened due to snow, the erroneous detection may occur. This is because when the snow melts, the flat surface may not be flat and thereby the vehicle tilts. In this case, because the vehicle tilts, the acceleration (tilt angle) detected by the acceleration sensing device exceeds the threshold value. Thus, it is determined that the vehicle is under the theft. Also, when the vehicle is quickly jacked up, it takes time before it is finally determined that the vehicle is under the theft because of a computation process for determining whether the acceleration (tilt angle) detected by the acceleration sensing means exceeds the threshold value.